Along with a recent increase in communication speed and terminal processing speed, there has been a need to raise the frequency of a reference oscillator. A voltage-controlled crystal oscillator using a high-frequency crystal resonator is strongly desired to reduce the drive level (the excitation level) of the crystal resonator.
FIG. 16 is a diagram showing the configuration of a general crystal oscillator. Referring to FIG. 16, a crystal oscillator comprises a crystal resonator SS and an oscillation circuit section CC for oscillating the crystal resonator SS. The oscillation circuit section CC includes an amplifier A and a resistor R which are connected in parallel to the crystal resonator SS, a load capacitive element Ca (with a capacitance value CCa) which is connected between the input side of the amplifier A and the ground, and a load capacitive element Cb (with a capacitance value CCb) which is connected between the output side of the amplifier A and the ground. The resistor R is also called a feedback resistor and is intended to define input and output DC operating points.
If the load capacitive element Ca and the load capacitive element Cb in this configuration are both variable capacitive elements, the oscillation frequency can be controlled. Here, let gm be the amplification factor of the amplifier A and Vxtal be the crystal voltage amplitude of the crystal resonator SS.
The configuration in FIG. 16 is represented as an equivalent circuit in FIG. 17. In FIG. 17, a crystal resonator side SSS is configured such that a crystal equivalent series capacitive component C1 (with a capacitance value CC1), a crystal equivalent series resistive component R1 (with a resistance value RR1), and a crystal equivalent series inductive component L1 (with a reactance value LL1) are connected in parallel to an inter-crystal-terminal capacitance C0 (with a capacitance value CC0). On the other hand, an oscillation circuit section side CCS is configured such that a resistive component Rn (with a resistance value RRn) and a capacitive component CL (with a capacitance value CCL) are series-connected. The resistive component Rn is a negative resistive component with a minus value. The resistance value RRn of the negative resistive component Rn counteracts the resistance value RR1 of the resistive component R1, which allows provision of a well-known LC oscillator.
Note that the capacitive component CL is an oscillator equivalent capacitive component of the equivalent circuit. The relationship between the capacitance value CCL of the capacitive component CL and the capacitance values CCa and CCb of the load capacitive elements Ca and Cb is given by expression (1):CCL=(CCa×CCb)/(CCa+CCb)  (1)
According to expression (2), the drive level P is directly proportional to the square of a frequency f. For this reason, if a crystal resonator operating in a high frequency band is used as the crystal resonator, the drive level P has a large value. The resistance value RRn of the negative resistive component Rn indicating the margin for oscillation of the circuit is given by expression (3):RRn=gm/{CCa×CCb×(2πf)2}  (3)
Referring to expression (3), the resistance value RRn of the negative resistive component Rn is inversely proportional to the square of the frequency. The absolute value of the resistance value RRn decreases with an increase in the frequency f. In normal design, an amplification factor gm is increased in order to increase the negative resistance. If the amplification factor gm is increased, a crystal voltage amplitude Vxtal generally increases to a power supply level, and the drive level P of the crystal resonator increases. The increased drive level P causes a problem such as shorter life of the crystal resonator.
In the case of a voltage-controlled crystal oscillator, an increased amplification factor gm makes it difficult to expand a variable oscillation frequency range. This point will be described below.
A voltage-controlled crystal oscillator is configured as shown in, e.g., FIG. 18. In FIG. 18, a load capacitive element Ca and a load capacitive element Cb are both variable capacitive elements. Controlling the capacitance of each variable capacitive element by a control voltage allows provision of a well-known voltage-controlled oscillator. More specifically, the capacitances are increased to lower the frequency and are reduced to raise the frequency. Note that, in FIG. 18, the load capacitive element Ca and the load capacitive element Cb each have a parasitic capacitance connected in parallel thereto (a dashed portion in FIG. 18).
An equivalent circuit of the configuration in FIG. 18 is as shown in FIG. 19. In FIG. 19, a capacitive component CL is an oscillator equivalent capacitive component of the equivalent circuit. The relationship between a capacitance value CCL of the oscillator equivalent capacitive component and an oscillation frequency f is given by expression (4):f=½π{LL1×CC1×(CC0+CCL)/(CC0+CC1+CCL)}1/2  (4)
For the sake of simplicity, let fL be a quantity that is a representation of the oscillation frequency f as a ratio. The quantity fL is given by expression (5):fL=(f−fs)/fs  (5)
In expression (5), a frequency fs is a series resonance frequency of a crystal resonator SS and is given by fs=½π(LL1×CC1)1/2.
Substitution of the oscillation frequency f and the series resonance frequency fs into expression (5) and approximation of the substitution result yield expression (6):
                                                        fL              =                            ⁢                              [                                                                            1                      /                      2                                        ⁢                    π                    ⁢                                                                  {                                                                                                            L                                                              L                                ⁢                                                                                                                                  ⁢                                1                                                                                      ·                                                          C                                                              C                                ⁢                                                                                                                                  ⁢                                1                                                                                      ·                                                                                          (                                                                                                      C                                                                          C                                      ⁢                                                                                                                                                          ⁢                                      0                                                                                                        +                                                                      C                                    CL                                                                                                  )                                                            /                                                                                ⁢                                                      (                                                                                          C                                                                  C                                  ⁢                                                                                                                                          ⁢                                  0                                                                                            +                                                              C                                                                  C                                  ⁢                                                                                                                                          ⁢                                  1                                                                                            +                                                              C                                CL                                                                                      )                                                                          }                                                                    1                        /                        2                                                                              -                                                                                                                                        ⁢                                                      1                    /                    2                                    ⁢                                                            π                      ⁡                                              (                                                                              L                                                          L                              ⁢                                                                                                                          ⁢                              1                                                                                ·                                                      C                                                          C                              ⁢                                                                                                                          ⁢                              1                                                                                                      )                                                                                    1                      /                      2                                                                      ]                            /                              {                                                      1                    /                    2                                    ⁢                                      π                    ⁡                                          (                                                                        L                                                      L                            ⁢                                                                                                                  ⁢                            1                                                                          ·                                                  C                                                      C                            ⁢                                                                                                                  ⁢                            1                                                                                              )                                                                      }                                                                                        =                            ⁢                                                                    {                                                                                            C                                                      C                            ⁢                                                                                                                  ⁢                            1                                                                          /                                                  (                                                                                    C                                                              C                                ⁢                                                                                                                                  ⁢                                0                                                                                      +                                                          C                              CL                                                                                )                                                                    +                      1                                        }                                                        1                    /                    2                                                  -                                  1                  ⁢                                                                          ⁢                  since                  ⁢                                                                          ⁢                  often                  ⁢                                                                          ⁢                                      C                                          C                      ⁢                                                                                          ⁢                      1                                                        ⁢                                      <<                                          (                                                                        C                                                      C                            ⁢                                                                                                                  ⁢                            0                                                                          +                                                  C                          CL                                                                    )                                                                                                                                              ≈                            ⁢                                                1                  /                  2                                ·                                  {                                                            C                                              C                        ⁢                                                                                                  ⁢                        1                                                              /                                          (                                                                        C                                                      C                            ⁢                                                                                                                  ⁢                            0                                                                          +                                                  C                          CL                                                                    )                                                        }                                                                                        (        6        )            
The variable frequency range when the capacitance of the parasitic capacitances and the like excluding a variable capacitance is small will be compared with that when the capacitance is large.
FIG. 20 is a graph showing a change in the quantity fL that is a representation as a ratio of the oscillation frequency f with respect to the capacitance value CCL of the oscillator equivalent capacitive component CL.
Referring to FIG. 20, if the value of the capacitance of the oscillator equivalent capacitive component CL excluding the variable capacitance is small, the capacitance value CCL is also small, as seen from expression (1). The variable frequency range in this case is ΔfL1 in FIG. 20. On the other hand, if the value is large, the capacitance value CCL is also large, as seen from expression (1). The variable frequency range in this case is ΔfL2 in FIG. 20. That is, even if variable ranges ΔCCL of the capacitance value CCL of the oscillator equivalent capacitive component CL are equal, the variable frequency range is wider when the value of the capacitance excluding the variable capacitance is smaller. For this reason, expansion of the variable frequency range is difficult when the value of the capacitance excluding the variable capacitance is large.
In a high frequency band, an amplification factor gm is generally increased in order to increase a resistance value RRn of a negative resistive component of the circuit. The increase in the amplification factor gm requires an amplifier to be of a larger size, resulting in the higher capacitance of the parasitic capacitances. This makes it difficult to expand the variable oscillation frequency range.
A configuration for solving the above-described problems associated with a drive level P of a crystal resonator and a variable frequency range at high frequencies has been disclosed (see, e.g., JP2001-308641A). The configuration will be described with reference to FIG. 21.
FIG. 21 is an example of a general method for keeping a crystal voltage amplitude Vxtal down. The example is configured such that a diode D1 is connected to keep a crystal voltage amplitude Vxtal down. In the configuration in FIG. 21, an anode of the diode D1 is connected to an output end whereas a cathode of the diode D1 is connected to the ground.
In the circuit configuration in FIG. 21, since the crystal voltage amplitude Vxtal is determined by a forward voltage drop of the diode D1 connected as a clamp diode, the crystal voltage amplitude Vxtal can be reduced. Here, let Vf be the forward voltage drop of the diode D1. The crystal voltage amplitude Vxtal is given by expression (7):Vxtal=(1/√2)×Vf  (7)
As can be seen from expressions (2) and (7), a drive level P of a crystal resonator can be reduced. Note that the forward voltage drop Vf of the diode D1 is, for example, 0.8 [V].